Vapor generator vapor temperature control



- y 1966 w. a. SCHUETZENDUEBEL ETAL 3,261,332

VAPOR GENERATOR VAPOR TEMPERATURE CONTROL Filed June 4, 1964 3,261,332VAPOR GENERATOR VAPOR TEMPERATURE CONTROL Wolfram G. Schuetzenduebel,Avon, and Richard D. Hottenstine and William H. Clayton, Jr., Windsor,Conn., assignors to Combustion Engineering, Inc., Windsor, Conan, acorporation of Delaware Filed June 4, 1964, Ser. No. 372,556 14 Claims.(Cl. 122-479) This invention relates to supercritical pressure reheatvapor generators and to means for controlling the vapor temperaturetherein.

It is well known that maximum efliciency of steam power plantsconsistent with the capability of the apparatus can be attained when thesteam temperature of both the primary circuit and the reheat circuit iscontrolled at its maximum value. During low load operation, inparticular, the temperature of the steam leaving both the primary andreheat circuits tends to drop off, with there being more of a tendencyfor the reheat temperature to drop than the temperature of the primarysteam. This is due not only to the predilection of boiler designers tokeep reheat surface away from furnace radiation, but also to thedecrease in temperature of the steam delivered to the reheater at thelower loads. It is desirable to have not only means of temperaturecontrol for one of the temperatures, but also a steam temperaturecontrol which is effective .to balance the two steam temperatures. Whileany of the conventional modes of steam temperature control may be usedto control the primary steam temperature, it is the control of thereheat steam temperature or the balance of these two, and therefore therelative heat absorption of the two circuits with which our invention isconcerned.

Our invention in a narrower sense comprises the location of primaryheating surface, preferably an economizer section, in a stream ofcombustion gases with reheat heating surface being located downstreamtherefrom. Hot fluid is circulated from the furnace circuits throughthis economizer decreasing its effectiveness so that the heat pickup ofthe reheating section is increased.

It is an object of this invention to provide in a supercritical reheatvapor generator an improved means of steam temperature control which isparticularly effective at reduced load.

It is a further object to obtain this vapor temperature control in asupercritical pressure reheat unit by decreasing the effectiveness ofthe heating surface of a first circuit whereby more heat is thereforepicked up in the second circuit.

Other and further objects of the invention will become apparent to thoseskilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination of the elements of theinventive organization in such a manner as to attain the resultsdesired, as hereinafter more particularly set forth in the followingdetailed description of an illustrative embodiment, said embodimentbeing shown by the accompanying drawing wherein:

FIGURE 1 is a diagrammatic representation of a side elevation of asupercritical pressure reheat vapor generator, showing the location ofthe furnace section recirculating pump in the through-flow portion ofthe circuit; and

FIGURE 2 is a similar representation wherein the circulating pump forthe furnace section is located in the furnace recirculating line.

In the illustrative embodiment of FIGURE 1 fuel and air are supplied tothe furnace or combustion chamber United States Patent "ice 2 burningthe fuel therein, passing gaseous combustion products through horizontalflue 4 and vertical flue 6 with the gases egressing through duct 8 to beconveyed through an air heater to the stack (not shown). Feedwater issupplied at high pressure to the vapor generator through conduit 10passing through low temperature economizer 12 and thereafter beingconveyed through the high temperature economizer 14. This heatedfeedwater is then conveyed through conduit 16 to the mixing vessel 18from which it is passed .through the furnace section 20. The primaryfluid passing through this section is heated by furnace radiation to ahigh temperature in the order of 800 F. and is conveyed from the outletpipe 22 to the finishing superheater section 24, where it is furtherheated by the combustion gases passing thereover through flue 4. Thissuperheated vapor is conveyed through outlet pipe 26 to a high pressureturbine (not shown).

The steam returning from this turbine enters as low temperature reheatsteam through conduit 28 passing through a low temperature reheatersection 30 and a high temperature reheater section 32. The reheatedsteam at high temperature is then conveyed through reheat outlet pipe 34to the reheat turbine (not shown).

Furnace section circulating pump 36 is located in the through-flowportion of the furnace heating section and is operative to inducerecirculation of hot fluid from the furnace section outlet pipe 22through the recirculating line 38 to the mixing vessel 18. The basicrecirculating system is described in US. Patent No. 3,135,252 to W. W.Schroedter. When pump 36 is operating the recirculated fluid throughline 38 mixes with the incoming preheated feedwater passing through line16 mixing in the mixing vessel 18. The fluid being pumped by circulatingpump 36 is, therefore, at a relatively .high temperature.

As the gaseous products of combustion or flue gas pass through fines 4and 6, they are cooled at each successive section by transferring heatto the heating surfaces located therein. It is evident, therefore, thatthe gas temperature entering each section is a function of the heatabsorbed in the preceding sections in relation to the gas flow.

In order to increase the reheated steam temperature egressing from thisunit, valve 48 is opened permitting a portion of the flow from pump 36to pass through line 42 mixing with incoming feedwater in line 13. Thisoperates to increase the fluid temperature entering the heating surfaceof the high temperature economizer 14 thereby decreasing the temperaturehead between the fluid heating surface 14 and the flue gases passingthereover. Because of this decreased temperature head less heat ispicked up by the high temperature economizer section 14 and the gastemperature leaving this section is consequently higher. This higher gastemperature passing over the reheater section 30 results in a highertemperature r head or therefore more heat picked up in the reheatsection. A portion of the excess heat in the gases entering the reheatsection 30 remains in the gases leaving that section such that the gastemperature entering the low temperature economizer 12 is alsoincreased. This is operative to increase the heat absorption in thatsection and therefore the fluid temperature passing through line 13,further emphasizing the increased fluid temperature entering the hightemperature economizer 14.

The higher reheat steam temperature leaving the low temperature reheater30 also enters the high temperature reheater 32 resulting in a higherreheat temperature leaving through the reheat outlet line 34.

Reheat outlet temperature sensing means 48 sends a control signal whichis matched with the desired reheater outlet temperature at set pointsetter 50 whereby an error signal is sent to controller 52. This signalis representative of a desired temperature entering the high temperatureeconomizer 14. This temperature entering the high temperature economizeris sensed by temperature sensing device 54 which sends a control signalto controller 52. The desired and measured temperatures are comparedwith this controller and the control signal sent through line 56 tooperate valve 40 so as to obtain desired temperature entering the hightemperature econornizer 14 and consequently the desired reheater outlettemperature.

This mechanism is therefore operative to vary the relative heatabsorption of the primary and reheat circuits, and may be controlled tobalance the primary and reheat outlet temperatures. Where our inventionis controlled to balance temperatures, conventional steam temperaturecontrol may be used to control the temperature level. Our inventionfunctions in the same manner, regardless of the system used to controlit. For instance, the valve 40 may be controlled to equalize the primaryand heat temperatures, with gas recirculation operating to maintain theprimary or reheat steam temperature. Alternately valve 40 may becontrolled to maintain the desired reheat temperature, with gasrecirculation being controlled to maintain the desired primarytemperature.

In order to make this control of maximum effectiveness, the surfaceshould be arranged so that during operation without reheat steamtemperature control the reheat section 30 operates at a very lowtemperature head, that is, the difference between the temperature of thegases in the flue 6 and the temperature of the reheat steam in thereheater 30 is very low. Although this will result in a high surfacerequirement for that section of the reheater, due to the lowtemperatures involved and the low pressure, it is relatively inexpensiveheating surface. Due, however, to this low temperature head, any changein the gas temperature entering the reheat section will be veryeffective in changing the amount of heat absorbed.

Due to the decreased pickup in the economizer 14 additional heat must beabsorbed elsewhere in the primary circuit. As previously described, aportion of this heat requirement is achieved in the low temperatureeconomizer 12. If an air heater is used in conjunction with this unit,the increased gas temperature entering the air heater results in anincreased preheated air temperature and, therefore, an increased heatrelease rate in the furnace. Additional increase as required must bemade up by firing additional fuel, which must be done in any event inorder to supply the additional heat being picked up now by the reheater.

The low temperature economizer 12 is not essential to the operation ofour invention although it is very helpful in attaining maximum steamgenerator efiiciency. This heating section aids in reducing the flue gastemperature to avoid excessive stack losses and contributes to theincrease in temperature entering section 14 thereby increasing theefficacy of the reheat steam temperature control.

It should furthermore be noted that the contribution of the hightemperature economizer 14 is such as to make the arrangement effectivefor reheat temperature control even if the reheat section 30 were notplaced downstream of the low temperature economizer 14. In such anarrangement passing the hot fluid through conduit 42, thereby increasingthe temperature. of the fluid entering the heat exchange section 14,decreases the heat pickup in that section. This varies the relative heatabsorption between the primary and reheat circuits. The decrease in heatabsorption of the primary circuit must then be made up elsewhere, inorder to maintain full steam temperature of the primary fluid. Anincrease in the furnace heat release would occur either by an increasedair heater temperature, by increased fuel input or by both. This, inturn, increases the gas temperatures throughout the unit resulting in anincreased gas temperature entering the reheat section 32 and anincreased heat absorption therein.

It can therefore be seen that although the instant embodiment would beoperative Without the inclusion of the low temperature reheat section30' or the low temperature economizer 12, the efiicacy of this mode ofsteam temperature control is substantially increased by theirincorporation. Regardless of whether heat exchange sections 30 and 12are included, reheat temperature control is obtained by regulating valve40 to vary the flow of the hot fluid passing through conduit 42 to heatfeedwater entering the high temperature economizer 14. The regulation ofvalve 40 is performed in such a manner as to control the temperature ofthe reheat steam leaving through steam pipe 34.

It should be noted that in the embodiment of this invention the heatused to increase the temperature of the fluid entering the hightemperature economizer is obtained from another part of the primarycircuit; namely, the furnace wall portion. The importance of this as anaid to the efficacy of the steam temperature control can be bestillustrated by considering the converse. If the temperature of the fluidentering the high temperature economizer were heated by something suchas a separately fired heating unit, the effectiveness of the economizerwould, of course, be decreased. However, since heat is being supplied tothe primary circuit in this heater, less heat need be supplied in thefurnace and therefore the firing rate of the unit would be decreased.This would, in turn, have the effect of decreasing the reheattemperature which tends to counteract the increased reheat temperaturewhich we are trying to obtain by increasing the temperature of the fluidin the high temperature economizer.

The steam generator illustrated in FIGURE 2 is similar to that of FIGURE1 except for the location of the recirculating pump 36. Therecirculating pump in this embodiment is located in the recirculatingline 38 whereby it pumps fluid from the furnace section outlet pipe 22to the mixing vessel 18. Reheat temperature control is obtained byregulating valves 44 and 46 to produce flow through conduit 42 therebyincreasing the temperature in the high temperature economizer 14. Theautomatic control signal operating through line 56 of FIGURE 1 mustaccordingly be modified to operate valves 44 and 46. Inasmuch as thefluid passing through line 42 in this embodiment is that leaving thefurnace section, it is of a considerably higher temperature than thefluid passing through conduit 42 of FIGURE 1. In FIGURE 1 the fluidpassed through this conduit is a mixture of the very high temperaturefluid and the incoming feedwater passing through conduit 16. The amountof fluid therefore which must pass through conduit 42 of FIGURE 2 inorder to obtain an effective control is somewhat less than the amountrequired in the embodiment of FIG- URE 1.

In a sub-critical unit a temperature of the fluids in sections such asthose with which we are here concerned are limited to the saturationtemperature of the Water therefore in such subcritical units the mode ofrecirculation such as herein described would not be effective tosubstantially change the temperature of the fluid entering the hightemperature economizer 14 and would therefore be ineffective for steamtemperature control. Our invention would however be effective to controlsteam temperature on a double reheat unit.

While we have illustrated and described a preferred embodiment of ourinvention it is to be understood that such is merely illustrative andnot restrictive and that variations and modifications may be madetherein without departing from the spirit and scope of the invention. Wetherefore do not wish to be limited to the precise details set forth butdesire to avail ourselves of such changes as fall within the purview ofour invention.

What We claim is:

1. A supercritical pressure vapor generator having a first circuit, asecond circuit, fluid passing through each circuit, a combustion zonefor the burning of fuel therein, means for conveying combustion gasesfrom said combustion zone; means efifective to transfer heat from saidcombustion gases to said first circuit, including a section of saidfirst circuit located in the flow path of said combustion gases; meanseffective to transfer heat from said combustion gases to said secondcircuit; means for transferring heat to the fluid entering said sectionof said first circuit from a portion of said first circuit downstream,with respect to the fluid flow, of said section of said first circuitwhereby the effectiveness of said section for heat absorption isreduced, thereby increasing the relative heat absorption of said firstcircuit; means for determining the relative heat absorption of thefluids flowing through said first and second circuits; and means forregulating said heat transferred to said section of said first circuitfrom the downstream portion of said first circuit in response to saidlast-named means.

2. An apparatus as in claim 1 wherein said first circuit includes tubeslining the walls of said combustion zone, a mixing vessel and means forconveying fluid from said section to the mixing vessel, means forconveying fluid from the mixing vessel to and through said tubes, meansfor conveying at least a portion of the fluid passing through said tubesto the mixing vessel; a pump located in the last-named means; said meansfor transferring heat to the fluid entering said section comprisingmeans for conveying fluid from the discharge of said pump to the inletof said section; and throttling means located between said pump and saidmixing vessel; and said means for regulating said heat transfercomprising throttling means located between said pump and said section.

3. An apparatus as in claim 1 wherein said first circuit includes tubeslining the walls of said combustion zone, a mixing vessel and means forconveying fluid from said section to the mixing vessel, means forconveying fluid from the mixing vessel to and through said tubes, meansfor conveying at least a portion of the fluid passing through said tubesto the mixing vessel; a pump located between said mixing vessel and saidtubes; said means for transferring heat to said section comprising meansfor conveying fluid from said pump discharge to said section, and saidmeans for regulating said heat transfer comprising throttling meanslocated in said means for conveying fluid to said section.

4. A supercritical vapor generator having a first circuit and a secondcircuit, fluid passing through each circuit, a combustion zone for thecombustion of fuel therein, and means for conveying a stream ofcombustion gases from said furnace; heating surface comprising a portionof said first circuit forming a section in the gas stream at an upstreamlocation; and heating surface comprising at least a portion of saidsecond circuit forming heating surface located in the gas stream at adownstream location; means for increasing the temperature of the fluidentering said section of said first circuit by transferring heat theretofrom a downstream portion of said first circuit with respect to fluidflow whereby the heat absorbing effectiveness of said section isdecreased, thereby increasing the relative heat absorption of said firstand second circuits; means for determining the relative heat absorptionbetween said first and second circuits; and means for regulating thetemperature of the fluid entering said section of said first circuit inresponse to said last-named means.

5. An apparatus as in claim 4 wherein said first circuit includes tubeslining the walls of said combustion zone, a mixing vessel and means forconveying fluid from said section'of said first circuit to the mixingvessel, means for conveying fluid from the mixing vessel to and throughsaid tubes, means for conveying at least a portion of the fluid passingthrough said tubes to the mixing vessel; a pump located in thelast-named means; said means for increasing the temperature of the fluidentering said section comprising means for conveying fluid from thedischarge of said pump to the inlet of said section of said firstcircuit; and said means for regulating the heat transfer comprisingthrottling means located between said pump and said mixing vessel, andthrottling means located between said pump and said section.

6. An apparatus as in claim 5 wherein said first circuit includes also alow temperature economizer portion, iocated upstream of said section ofsaid first circuit with relation to fluid flow, but located downstreamwith respect to gas flow, and also located downstream with respect togas flow of said heating surface comprising a portion of said secondcircuit.

7. An apparatus as in claim 4 wherein said first circuit includes tubeslining the walls of said combustion zone, a mixing vessel and means forconveying fluid from said section of said first circuit to the mixingvessel, means for conveying fluid from the mixing vessel to and throughsaid tubes; means for conveying at least a portion of the fluid passingthrough said tubes to the mixing vessel; a pump located between saidmixing vessel and said tubes, said means for transferring heat to saidsection of said first circuit comprising means for conveying fluid fromsaid pump discharge to said section; and said means for regulating saidheat transfer comprising throttling means located in said means forconveying fluid to said section.

8. An apparatus as in claim 7 wherein said first circuit includes also alow temperature economizer portion, located upstream of said section ofsaid first circuit with relation to fluid flow, but located downstreamwith respect to gas flow, and also located downstream with respect togas flow of said heating surface comprising a portion of said secondcircuit.

9. A supercritical vapor generator having a primary circuit with fluidflowing therethrough, a reheat circuit with the fluid flowingtherethrough, a furnace for the combustion of fuel therein, and a fluefor the conveyance of combustion gas therefrom; a primary section ofheating surface comprising a portion of said primary circuit located insaid flue at an upstream location with respect to the gas flow; areheater section comprising at least a portion of said reheater circuitlocated in said flue at a downstream location with respect to the gasflow; means for transferring heat to the fluid entering said section ofthe primary circuit from a portion of the primary circuit downstream ofsaid primary section with respect to fluid flow; means for determiningthe temperature of the fluid leaving the reheat circuit; and means forregulating said means for transferring heat to the fluid entering saidprimary section in response to said tem perature determining means.

10. A supercritical pressure vapor generator having a primary circuit, areheat circuit, a furnace for the combustion of fuel therein, a flue forthe conveyance of combustion gases therefrom; a first section of primaryheating surface comprising a portion of said primary circuit, located insaid flue at an upstream location with respect to the gas flow; a secondsection of primary heating surface comprising tubes lining the furnacewalls; a reheater section of heating surface comprising at least aportion of said reheater circuit located in said flue at a downstreamlocation with respect to the gas flow; means for conveying lflllld fromsaid first section to said furnace section during normal operation; andmeans for returning a portion of the fluid from said furnace section tosaid first section to increase the temperature of the fluid pas-singtherethrough; measuring means for determining a measure of the heatabsorption in said reheat circuit; and means for regulating the quantityof fluid returned to said first section in response to said measuringmeans.

11. An apparatus as in claim 5 including also a third heating surfacecomprising a portion of said primary circuit, said third section locatedupstream of said first section with relation to the fluid flow butlocated downstream of said first and second sections with respect to thegas flow.

- 12. The method of operating a vapor generator comprising; burning fuelin a combustion zone and forming products of combustion, passing a fluidat relatively high pressure in convection heat exchange with theproducts of combustion in a convection zone, thereafter passing said[fluid at high pressure in heat exchange relation with the products ofcombustion in another zone, passing a lower pressure fluid in heatexchange relation with the products of combustion, returning a portionof the high pressure fluid from said other zones and mixing saidreturned fluid with the high pressure fluid entering said convectionzone whereby the temperature of the fluid entering said convectionzone'is increased and the heat absorption in this Zone is decreased,determining the relative heat absorption of the high pressure and lowpressure fluids, and regulating the amount of fluid returned in responseto the determined relative heat absorption.

13. The method of operating a vapor generator comprising burning fuel ina combustion zone and forming products of combustion passing a fluid atrelatively high pressure in convection heat exchange with the productsof combustion in a convection zone at an upstream location, thereafterpassing said fluid at high pressure in heat exchange relation with theproducts of combustion in another zone, passing a lower pressure fluidin heat exchange relation with the products of combustion includingpassing the lower pressure fluid in heat exchange relation with theproducts of cornbustion at a downstream location, returning a portion ofthe high pressure fluid from said other zone and mixing said returnedfluid with the high pressure fluid entering said convection zone at saidupstream location whereby the temperature of the fluid entering saidconvection zone is increased and the heat absorption in this zone isdecreased, the gas temperature entering the downstream locationtherefore being increased and the heat absorption at said downstreamlocation being increased, determining the relative heat absorption ofthe high and low pressure fluids, and regulating the amount of fluidreturned in response to the determined relative heat absorption.

14. The method of operating a vapor generator comprising burning fuel ina combustion zone and forming products of combustion passing a fluid atrelatively high pressure in heat exchange relation with the products ofcombustion at a downstream location, subsequently passing said fluid inconvection heat exchange with the products of combustion in a convectionzone at an upstream location, thereafter passing said fluid at highpressure in heat exchange relation with the products of combustion inanother zone, passing a lower pressure fluid in heat exchange relationWith the products of combustion including passing the lower pressurefluid in heat exchange relation with the products of combustion at alocation intermediate said downstream and upstream locations, returninga portion of the high pressure fluid from said other zone and mixingsaid returned fluid with the high pressure fluid entering saidconvection zone at said upstream location whereby the temperature of thefluid entering said convection zone is increased and the heat absorptionin this zone is decreased, the gas temperature entering the downstreamlocation therefore being increased and the heat absorption by the lowpressure fluid passing through said intermediate location beingincreased, determining the relative heat absorption of the high and lO'Wpressure fluids, and regulating the amount of fluid returned in responseto the determined relative heat absorption.

References Cited by the Examiner UNITED STATES PATENTS 3,038,453 6/1962Armacost 122-406 3,135,246 6/1964 Kochey l22479 3,186,175 6/1965Strohmeyer l22479 KENNETH W. SPRAGUE, Primary Examiner.

1. A SUPERCRITICAL PRESSURE VAPOR GENERATOR HAVING A FIRST CIRCUIT, ASECOND CIRCUIT, FLUID PASSING THROUGH EACH CIRCUIT, A COMBUSTION ZONEFOR THE BURNING OF FUEL THEREIN MEANS FOR CONVEYING COMBUSTION GASESFROM SAID COMBUSTION ZONE; MEANS EFFECTIVE TO TRANSFER HEAT FROM SAIDCOMBUSTION GASES TO SAID FIRST CIRCUIT, INCLUDING A SECTION OF SAIDFIRST CIRCUIT LOCATED IN THE FLOW PATH OF SAID COMBUSTION GASES; MEANSEFFECTIVE TO TRANSFER HEAT FROM SAID COMBUSTION GASES TO SAID SECONDCIRCUIT; MEANS FOR TRANSFERRING HEATING TO THE FLUID ENTERING SAIDSECTION OF SAID FIRST CIRCUIT FROM A PORTION OF SAID FIRST CIRCUITDOWNSTREAM, WITH RESPECT TO THE FLUID FLOW, OF SAID SECTION OF SAIDFIRST CIRCUIT WHEREBY THE EFFECTIVENESS OF SAID SECTION FOR HEATABSORPTION IS REDUCED, THEREBY INCREASING THE RELATIVE HEAT ABSORPTIONOF SAID FIRST CIRCUIT; MEANS FOR DETERMINING THE RELATIVE HEATABSORPTION OF THE FLUIDS FLOWING THROUGH SAID FIRST AND SECOND CIRCUITS;AND MEANS FOR REGULATING SAID HEAT TRANSFERRED TO SAID SECTION OF SAIDFIRST CIRCUIT FROM THE DOWNSTREAM PORTION OF SAID FIRST CIRCUIT INRESPONSE TO SAID LAST-NAMED MEANS.